Polar low tracks over the Nordic Seas: a 14-winter climatic analysis

Abstract

A B S T R A C TTo develop a 14-winter (OctoberÁApril; 1999Á2013) climatic description of polar low (PL) occurrence for the Nordic Seas, systems have been tracked using images acquired from the Very High Resolution Radiometer (AVHRR). Also, the dominant PL characteristics Á their temporal and spatial distributions, size, lifespan, distance travelled, speed of propagation and directions Á have been determined. On average, 14 PL events occur per winter but there is strong inter-annual and intra-seasonal variability. Although systems may form and travel over the whole Nordic Seas, their genesis is enhanced in areas characterised by warm oceanic currents. At the start of the season (OctoberÁNovember), systems mainly form over the Greenland and Norwegian Seas, but further into winter they form increasingly over the Barents Sea. In connection with recent low-ice winters, new areas of PL formation are evident, particularly to the west of Spitsbergen and in the Barents Sea. PL speeds of propagation range between 5 and 13 m/s but are observed to be highly variable among cases and even during the lifespan of individual PLs. To a considerable extent, the direction of movement is controlled by the large-scale flow in the lowest atmospheric layers, but we also observed cyclonic co-rotation of some pairs of PLs due to their influence on the ambient flow. Although these generally move southward or southeastward, a substantial number of PLs have westward and even northward tracks. PLs in the western part of the region average larger than their eastern counterparts. This study also highlights that PLs characteristics and tracks differ according to weather regimes.

An objective global climatology of polar lows based on reanalysis data

“…The STARS dataset version 2 provides a list of 185 PL tracks over the Nordic Seas from January 2001 to March 2011 (Noer et al , ). The PLs are subjectively identified by forecasters at the Norwegian Meteorological Institute by inspection of satellite infrared data, scatterometer winds and the operational weather forecasting model HIRLAM 4 (Rojo et al , ; Terpstra et al , ), for evaluation of PL occurrence in different datasets (e.g. (Laffineur et al , )(Smirnova and Golubkin, )), and the evaluation of objective detection methods (e.g (Zappa et al , )).…”

Arctic warming, moisture increase and circulation changes observed in the Ny-Ålesund homogenized radiosonde record

“…Analysing the difference in daily AO indices between the periods 1993-2003 and 2004-2014, we also find a shift towards more negative AO indices. Furthermore, a recent increase of polar lows over the Greenland Sea-west of Svalbard-and over the Barents Sea has been found that may partly be explained by a change in the seasonal evolution of sea ice extent (Rojo et al 2015).…”

“…Preisendorfer 1988;Hannachi et al 2007) both for the winter season (DJF) and the summer season (JJA). On this basis, the daily AO indices were obtained by projecting the daily mean sea-level pressure anomalies poleward of 20°N onto the winter and summer AO patterns, respectively.…”

In Situ Measurements of Surface Winds, Waves, and Sea State in Polar Lows Over the North Atlantic

“…Satellite-derived ocean WS data are not utilized here because the cloud liquid water content typically accompanying PLs adversely affects the accuracy of the retrievals (Claud et al, 1993;Draper & Long, 2004;Hilburn et al, 2006;Moore et al, 2008;Weissman & Bourassa, 2008;Renfrew et al, 2009;Weissman et al, 2012). Manual tracking of PLs on the satellite images is used (after Rojo et al, 2015) rather than an automated detection algorithm applied to reanalysis data because of the documented problem of poor or inconsistent representations of PLs in reanalyses (Laffineur et al, 2014;Zappa et al, 2014).…”